This invention relates to the casting of metal strip. It has particular but not exclusive application to the casting of ferrous metal strip.
It is known to cast metal strip by continuous casting of strip in a twin roll caster.
Molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term xe2x80x9cnipxe2x80x9d is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or a series of smaller vessels from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow, although alternative means such as electromagnetic barriers have also been proposed.
Although twin roll casting has been applied with some success to non-ferrous metals which solidify rapidly on cooling, there have been problems in applying the technique to the casting of ferrous metals which have high solidification temperatures and a tendency to produce defects caused by uneven solidification at the chilled casting surfaces of the rolls.
Strip defects such as, for example, severe transverse cracks may cause the strip to tear and shear and may, during continuous casting, cause a complete shutdown of operations if adequate provision is not made for effective handling of defective strip on line. Malfunctioning of the casting or strip handling equipment can also require a complete shutdown of the casting line.
Shut downs of continuous strip casting operations are expensive in terms of loss of productivity, in terms of the potential for damage of equipment and in terms of the increase in risk to the health and safety of personnel and other hazards that occur as a result of strip breakout.
The present invention enables shut downs to be minimised, even when defective strip develops or there is an equipment failure, by allowing diversion of the defective strip to scrap and subsequent take up of strip for end use when the cause of the defects or equipment malfunction has been rectified, without shutting down the continuous strip casting operation.
According to the invention there is provided a method of continuously casting and handling metal strip, comprising:
supporting a casting pool of molten metal on one or more chilled casting surfaces;
moving the chilled casting surface or surfaces to produce a solidified strip moving away from the casting pool;
guiding the solidified strip along a transit path which takes it away from the casting pool to a coiling station;
coiling the strip onto a coiler at the coiling station;
operating a strip shearing means so as to sever the strip in advance of the coiling station into discrete lengths;
diverting the severed lengths of strip from the coiling station to a scrap station; and
halting the operation of the strip shearing means so as to reinstate delivery of continuous strip to the coiling station and directing the reinstated continuous strip to a second coiler at the coiling station.
Preferably, a scrap bin is located at the scrap station to receive the severed lengths of strip.
The operation of the strip shearing means and diversion of the severed lengths of strip to the scrap station may be activated in response to observation or detection of defects in the strip or malfunctioning of equipment employed in the casting or handling of the strip. In this context the term xe2x80x9cdefectsxe2x80x9d may extend to any properties or attributes of the strip which may be undesirable for an end user or for further processing. Such defects may be observed or detected by visual observation by an operator and/or by detection instrumentation.
Specifically, the method may include the steps of inspecting the strip for defects as it passes in said transit path by means of strip defect detection means located along that path and initiating operation of the strip shearing means in response to an indication from the defect detection means of defects in the strip.
The defect detection means may be operated to detect variations in thickness of the strip and/or surface defects in the strip.
The defect detection means may comprise an X-ray gauge and a surface defect detector.
The casting pool of molten metal may be maintained by flow of molten metal from a tundish supplied alternatively by each of a plurality of ladles brought to a pouring position by a rotating turret.
The invention further provides apparatus for continuously casting and handling metal strip comprising:
a pair of generally horizontal casting rolls forming a nip between them;
metal delivery means to deliver molten metal into the nip between the casting rolls to form a casting pool of molten metal supported on the rolls;
means to chill the casting rolls;
means to rotate the casting rolls in mutually opposite directions whereby to produce a cast strip delivered downwardly from the nip;
strip guide means to guide the strip delivered downwardly from the nip through a transit path which takes it away from the nip;
a coiling station to receive strip from said transit path and provided with a pair of coilers and strip deflector means operable alternatively to direct the strip to one or other of the coilers;
strip shearing means operable repeatedly to sever the strip in advance of the coiling station to chop the continuously delivered strip into a multiplicity of discrete strip lengths; and
strip diverter means to divert the multiplicity of discrete strip lengths so as to bypass both coilers at the coiling station and deliver them to a scrap station.
The apparatus may further include strip defect detection means disposed along said transit path in advance of the coiling station and operable to detect defects in the strip and the shearing means may be operable in response to an indication from the defect detection means of defects in the strip.